Complex long‐term dynamics of pollinator abundance in undisturbed Mediterranean montane habitats over two decades

Abstract

Current notions of “pollinator decline” and “pollination crisis” mainly arose from studies on pollinators of economic value in anthropogenic ecosystems of mid‐latitude temperate regions. Comprehensive long‐term pollinator data from biologically diverse, undisturbed communities are needed to evaluate the actual extent of the so‐called “global pollination crisis.” This paper analyzes the long‐term dynamics of pollinator abundance in undisturbed Mediterranean montane habitats using pollinator visitation data for 65 plant species collected over two decades. Objectives are (1) to elucidate patterns of long‐term changes in pollinator abundance from the perspectives of individual plant species, major pollinator groups, and the whole plant community and (2) to propose a novel methodological implementation based on combining a planned missing data design with the analytical strength of mixed effects models, which allows one to draw community‐wide inferences on long‐term pollinator trends in species‐rich natural habitats. Probabilistic measurements (“patch visitation probability” and “flower visitation probability” per time unit) were used to assess pollinator functional abundance for each plant species on two separate, randomly chosen years. A total of 13,054 pollinator censuses accounting for a total watching effort of 2,877,039 flower‐min were carried out on 299 different dates. Supra‐annual unstability in pollinator functional abundance was the rule, with visitation probability to flowering patches and/or individual flowers exhibiting significant heterogeneity between years in the majority of plant species (83%). At the plant‐community level, there was a significant linear increase in pollinator functional abundance over the study period. Probability of pollinator visitation to flowering patches and individual flowers increased due to increasing visitation by small solitary bees and, to a lesser extent, small beetles. Visitation to different plant species exhibited contrasting changes, and insect orders and genera differed widely in sign and magnitude of linear abundance trends, thus exemplifying the complex dynamics of community‐wide changes in pollinator functional abundance. Results of this investigation indicate that pollinator declines are not universal beyond anthropogenic ecosystems; stress the need for considering broader ecological scenarios and comprehensive samples of plants and pollinators; and illustrate the crucial importance of combining ambitious sampling designs with powerful analytical schemes to draw reliable inferences on pollinator trends at the plant community level.

Species energy and Thermal Performance Theory predict 20‐yr changes in ant community abundance and richness

“…Another intensive study of protected fields and meadows yielded no 20-yr net decline in the abundance of insect pollinators (Herrera 2018). Another intensive study of protected fields and meadows yielded no 20-yr net decline in the abundance of insect pollinators (Herrera 2018).…”

Gradual replacement of wild bees by honeybees in flowers of the Mediterranean Basin over the last 50 years

“…Correlative and experimental evidence alike has recently shown that, at local or regional scales, honeybees can have strong negative impacts on wild bee populations in both natural and anthropogenic scenarios (Shavit et al 2009, Lindström et al 2016, Torné-Noguera et al 2016, Magrach et al 2017, Ropars et al 2019, Valido et al 2019), and that the absence of honeybees in well-preserved natural areas is associated with increasing wild bee populations (Herrera 2019). Much of the direct or circumstantial evidence on the harmful effects of honeybees on wild bees originated in the Mediterranean Basin, which motivated the hypothesis formulated in this paper of a possible replacement of wild bees by honeybees in the Mediterranean running parallel to the increasing abundance of honeybees.…”

“…S1, Potts et al 2010, vanEngelsdorp and Meixner 2010). Honeybees can have strong negative impacts on wild bee populations in both natural and anthropogenic scenarios (Shavit et al 2009, Lindström et al 2016, Torné-Noguera et al 2016, Herrera 2019, Ropars et al 2019, Valido et al 2019). I thus formulated the hypothesis that, if the abundance of managed honeybees has been actually increasing in the Mediterranean Basin over the last decades, then a profound biome-wide alteration in the composition of bee pollinator assemblages could be currently underway there, involving a progressive replacement of wild bees by honeybees in flowers.…”

“…Critical elements of ecological knowledge will thus be closely tied to the particular location where it is attained, and attempts at extrapolations which are based on limited, spatially biased ecological data may produce distorted or erroneous inferences (Martin et al 2012, Culumber et al 2019). For instance, unawareness of geographical sampling biases has been pointed out as one possible weakness of generalizations on “pollinator decline” and “pollination crisis” (Ghazoul 2005, Archer 2014, Herrera 2019, Jamieson et al 2019), two topics that have recently elicited considerable academic and societal interest because of the importance of animal pollination for the reproduction of many wild and crop plants (Ollerton et al 2014, Senapathi et al 2015, Breeze et al 2016, Ollerton 2017). Evidence for the view of a generalized pollinator decline is strongly biased geographically, as it mostly originates from a few mid-latitude regions in Europe and North America (Rodger et al 2004, Ghazoul 2005, Winfree et al 2009, Archer 2014, Hung et al 2018, Nicholson and Egan 2019).…”

Are insects declining and at what rate? An analysis of standardised, systematic catches of aphid and moth abundances across Great Britain

“…Even after the widespread habitat destruction during World War I, it is perhaps surprising that butterflies and moths saw a reversal in fortunes, although there are exceptions (Bretherton, 1951). Whilst it is rarely recognised or cited that insects may profit as a result of environmental change (Bell et al, 2015;Herrera, 2019, Boyes et al, 2019, Macgregor et al, 2019, it adds a dimension to the question of whether insects are declining and if so at what rate, because it suggests that to answer such a question will be dependent on the species or group studied, ignoring other likely important covariates such as habitat, spatio-temporal issues, statistical methodology and sampling intensity and bias, for example (McKinney, 1999;Cardoso et al, 2019;Mupepele et al, 2019;Simmons et al, 2019;Thomas et al, 2019).…”